This invention relates generally to tandem drive axle sets and, more particularly, to an inter-axle differential assembly for a tandem drive axle set.
A tandem drive axle set is used to distribute rotational power from a driveline input to a set of forward and rear wheels through a forward drive assembly and a rear drive assembly, respectively. Traditionally, the tandem drive axle set is designed such that the forward drive assembly has a ring gear and a pinion gear set that is a mirror image of the rear drive assembly ring gear and pinion gear set. Usually, the forward drive assembly has a right-hand pinion gear while the rear drive assembly has a left-hand pinion gear. It is necessary that the forward and rear drive assemblies be mirror images of each other because, traditionally, the forward drive assembly has included a set of helical gears that are used to transfer half of the rotational power from an inter-axle differential to the forward drive assembly pinion and ring gear set. That is, the forward and rear drives require distinct parts, which requires increased inventory.
The inter-axle differential receives rotational input from the driveline of the vehicle. In such a design, the pinion gear of the forward drive assembly rotates in the opposite direction to that of the inter-axle differential. The inter-axle differential transmits the other half of its input to a through shaft which sends the input back to the rear drive assembly. In the rear drive assembly the pinion gear rotates in the same direction as the inter-axle differential. Because the helical gears are necessary in the forward drive assembly, the axis of the input to the forward drive assembly is offset from the pinion gear axis in the forward drive assembly by the centerline-to-centerline distance of the helical gears. Therefore, the output of the forward axle is on the same axis as the input while the input of the rear drive assembly is on the same axis as the forward axle drive pinion gear.
This difference in axis height between the forward axle output to the rear axle input requires different axle pinion angles to be utilized in order to set the driveline angles in the u-joints used in the driveline. Setting and maintaining the driveline angles is difficult. When the driveline angles at the u-joints are not the same it creates adverse torsional loading and vibrations in the drivetrain assembly. Such torsional loading and vibrations can lead to premature failure of the drivetrain assembly. Even when the driveline angles are properly set at the factory, the air-ride suspensions commonly found in heavy duty trucks can alter the driveline working angles in an adverse manner.
Therefore, it is desirable to provide a tandem axle drive set wherein the input power to the forward drive assembly is set on the same axis as the input power to the rear drive assembly. This eliminates the need to set driveline angles and to provide distinct drive components.